Probing the Ultrafast Charge Translocation of Photoexcited Retinal in Bacteriorhodopsin

Almost all structural changes in proteins are preceded and accompanied by fast changes in the charge distribution of the system. While theory predicts these charge redistributions in protein for quite a while, there has so far not been any experiment that could detect them. The main reason has been that the charge redistributions in proteins occur on a very short time scale and existing voltmeter are by far too slow to follow these processes, let alone at the nano-scale.

Our strategy to measure charge redistribution in proteins is been based on two aspects. On the one hand, we used the amino-acid tryptophan as a natural in situ voltmeter. On the other hand, our highly sophisticated spectroscopic set-up provides the required high time resolution to follow these processes. With this approach, we obtained the first measured data on the initial charge rearrangement on the retinal chromophore in bacteriorhodopsin: The dipole moment of the retinal increases within 200 fs, which finally leads to the structural change of retinal.

This example emphasizes the performance of the new methodology to follow the fundamental process of charge translocation in a large class of biological systems - which is important for understanding and mimicking of protein function.